Does this mean you still have option in other versions(15.x) to toggle between classic and New UX and only issue in 15.4 , I thought users don't have option to go back to classic(not via tweaking url ) once New UX timesheet is switched On, is this not true?

Once you have checked “Activate New User Experience” in System Options you are able to use both the new and the classic timesheets, however until “Activate Timesheets” is marked you will not have a menu item for the timesheets in the new UI, it will only work from direct link. The same goes for the Classic UI; once you have marked “Activate Timesheets” the Classic UI timesheets can only be accessed from direct link, no menu will be shown for it.

Below you can see the links to both the Classic and New UI timesheet pages.

Link to timesheets in Classic: http://<server>/niku/nu#action:timeadmin.timesheetBrowserReturn

That option you have screen shot, will enable timesheet in the new UI. If you enable it, ALL users in the original UI will no longer have access to timesheets in the original UI.

However, your description of the issue experienced is not the above, as some staff have access to Timesheet in the Original UI, and some don't. There has to be something else preventing these staff from accessing Timesheet in the Original UI.